This invention pertains to a one-dimensional photon-counting detector array and in particular to such an array utilizing microchannel array plates and a multiple element anode.
Instruments for photometric studies at ultraviolet and x-ray wavelengths have traditionally been divided into two distinct classes: photographic and photoelectric. Photographic instruments, employing film as the detection system, have the great advantage of an image-storing capability. It is therefore possible to use this type of instrument to record a very large amount of data with a single exposure. However, photographic film has a number of major disadvantages. First, the sensitivity is considerably lower than that of a photoelectric detector; at ultraviolet wavelengths, where special emulsions are required, the quantum efficiency is typically about 1% that of a photoelectric detector. Second, the response is nonlinear as a function of the incident energy. Hence, photometric calibration is a difficult and time-consuming procedure, and the resulting accuracy is poor. Furthermore, the emulsion is sensitive to a very wide range of wavelengths; accordingly, the elimination of background fog levels is extremely difficult. Finally, the output signal is not electrical in character. The use of photographic film in space experiments on orbiting satellites thus implies either recovery of the film or a complex onboard processing and video transmission system.
Photoelectric instruments, on the other hand, are more sensitive, have a greater stability of response, and provide a linear output as a function of the incident energy. The output data format is also fully compatible with spacecraft data-handling and telemetry systems. However, since most photoelectric detectors do not have image-recording capabilities, the data must be read out sequentially, point-by-point. Consequently, the overall speed of the system is quite slow.
The SEC vidicon tube has been used extensively on sounding rockets and orbiting satellites to combine some of the advantages of both the photographic and the photoelectric detection techniques. Although this device has an image-recording capability and an electrical readout, it has major limitations in terms of dynamic range, stability of response, resolution, and photometric accuracy.
The development of the channel electron multiplier and its miniaturization into the microchannel array plate have been important developments in the field of photometrics, combining the advantages of both the photographic and the photoelectric detection systems. The microchannel array plate can be operated as an image intensifier and has the potential of being developed to yield signal outputs superior to those of conventional photomultipliers. In particular, the microchannel array plate has a photon-counting capability and a negligible dark count rate. These devices can operate stably and efficiently at extreme ultraviolet (EUV) and soft x-ray wavelengths in a windowless configuration or can be installed with a photocathode in a sealed tube for use at ultraviolet and visible wavelengths.
The readout systems generally employed with microchannel array plates in the prior art has been a visible-light phospor coupled to either a vidicon tube or photographic film. In this arrangement, the detected photon is converted to a pulse of electrons in the microchannel; these electrons are accelerated toward the phosphor and reconverted to visible photons, which are detected by either the vidicon photocathode or the photographic emulsion. Although the microchannel array plate can provide a gain on the order of 10.sup.7, this system is cumbersome and has all the inherent disadvantages of either the photographic plate or the vidicon tube.
In order to exploit the full sensitivity, dynamic range and photometric stability of the microchannel array plate, it is necessary to employ pulse-counting readout systems working directly at the anode. Some examples of pulse-counting systems to read out spatial information from microchannel array plates have been described in the prior art, but have been designed to employ a limited number of amplifiers, two for a one-dimensional array and four for a two-dimensional array, and have consequently been limited in terms of dynamic range and spatial resolution. This is especially the case for applications at high signal levels such as from laboratory EUV and soft x-ray sources or from telescopes for solar studies at EUV and soft x-ray wavelengths. While there is a suggestion of a multielement anode array in "The Multianode Photomultiplier", by Catchpole and Johnson, Pub. Astron. Soc. Pacific, Volume 84, February 1972, pages 134-136, there is no disclosure of any parameters for such a construction, nor of any data handling construction. In accordance with the present invention it has been found that a practical and workable multielement anode array can only be arrived at by careful control of the parameters, as fully discussed hereinafter.